Replacing the metal electrodes in triboelectric nanogenerators: High-performance laser-induced graphene electrodes

While significant work is being carried out to develop materials for enabling high surface charge density triboelectric nanogenerators (TENG), little attention has been paid to the role of electrodes responsible for charge collection. This work reports on the facile synthesis and use of high crystallinity, sp(2)-hybridised laser-induced graphene (LIG) as a high-efficiency electrode for triboelectric nanogenerators (TENGs). Using a conventional 10.6 mu m CO2 laser, the controlled, direct photothermal conversion of dielectric, tribo-negative polyimide (PI) and tribo-positive cellulosic paper into corresponding PI-LIG and paper-LIG, respectively, facilitates a significantly higher electrical output as compared to the commonly utilised adhesive aluminium electrodes. The LIG based paper-PI TENGs showed significantly higher electrical output characteristics with a peak-to-peak voltage of up to-625 V, a current density of-20 mA m(-2) and a transferred charge density of-138 mu Cm-2 with a maximum power output of-2.25 W m(-2), respectively, while the corresponding values for the conventional Al-tape electrode-based paper-PI TENGs were 400 Vp-p, similar to 10 mA m(-2), similar to 85 mu C m(-2) and 0.9 W m(-2), respectively. The mechanically robust LIG electrodes show excellent stability with less than 5.0% variation in output over 12,000 contact cycles. Using Kelvin probe force microscopy (KPFM) measurements, we have measured differences in not only the average surface potentials of the triboelectric surfaces (-0.26 V for pristine PI vs. +0.34 V for paper, which drive the TENG electrical output) but also for the LIG's synthesised from them (-0.08 V for PI-LIG vs. +0.26 V for paper-LIG), suggestive of the role of initial surface chemistry in the formation of LIGs. The enhanced (similar to 150%) power density for LIG based TENGs is ascribed to the lowering of the charge transfer barrier height via the alignment of Fermi levels resulting in higher surface charge on the dielectric surface and the significantly (similar to 6 orders) lower interfacial contact impedance of LIG as compared to adhesive aluminium electrodes. Thus, via the removal of the additional interface between the triboelectric surface and electrode, high-performance metal-free TENGs with excellent prospects for enabling energy harvesting applications can be realised.